In the related art, there is an inventory management system which uses RFID (Radio-Frequency Identification) tags for tracking individual articles such as commercial goods, books, or documents. The RFID tag attached to an article stores information, such as an identification number (ID) uniquely assigned to the article, which can be read by an RFID tag reader. When the RFID tag reader simultaneously reads information from a plurality of RFID tags attached to articles spread over a wide area, it may be necessary to identify a position or an area where each article is located. In the related art, there is a technique for detecting a distance and direction of the RFID tag from the RFID tag reader based on an intensity of radio waves received from the RFID tag by a directional antenna. However, in general, the intensity of the radio waves received from the RFID tag depends not only on its distance from the RFID tag reader but also on the RFID tag's relative orientation. For this reason, if the RFID tags on the articles are not arranged in a fixed orientation, it becomes difficult to accurately detect a distance from the RFID tag reader based on the intensity of the radio waves received through the antenna.